This paper describes the useful yields of positive B-11, Si-28, and Ge-70 ions produced from the surface of B-doped Si1-xGex alloys when the surface is irradiated by an 8 keV positive O-2 ion beam. Boron doping into alloys was done by implanting B-11 ions with an energy of 30 keV at a dose of 1x10(15) cm(-2). The reactive sensitivity factors (RSFs) of B-11 to the matrix isotopes of Si-28 Or Ge-70 are determined using the useful yields, and the relationships between the RSFs and the alloy compositions are shown. Also described are the RSFs of B-11 estimated from the local thermal equilibrium model with two internal standard elements of Si and Ge. The theoretical RSFs were a factor of 3-4 greater than the experimental RSFs.